do you need a blog to get a user name?

I would like to set my blog to "private" so only certain people I choose can read it. I found this under privacy settings. I noticed that you need to have a word press user name to add to the site which allows that person to gain access. Does this mean the people I want to view my blog must also have blogs on word press? Or is there a way for them to register and still be able to view my blog?
Thank you.
Blog url: http://fourpachecos.wordpress.com/